1. Field of the Invention
The present invention relates to an organic electroluminescent device, and more particularly, to an active matrix organic electroluminescent device including a thin film transistor.
2. Discussion of the Related Art
A cathode ray tube (CRT) has been widely used as a display screen for devices, such as televisions and computer monitors. However, a CRT has the disadvantages of being large, heavy, and requiring a high drive voltage. As a result, flat panel displays (FPDs) that are smaller, lighter, and require less power have grown in popularity. Liquid crystal display (LCD) devices, plasma display panel (PDP) devices, field emission display (FED) devices, and electroluminescence display (ELD) devices are some of the types of FPDs that have been introduced in recent years.
Among various types of FPDs, ELD devices use an electroluminescence phenomenon that light is emitted when a specific voltage is applied to a fluorescent material. ELD devices may either be an inorganic electroluminescence display device or an organic electroluminescence display (OELD) device depending upon the source material used to excite carriers in the device. OELD devices have been particularly popular because they have bright displays, low drive voltages, and can produce natural color images incorporating the entire visible light range. Additionally, OELD devices have a preferred contrast ratio because they are self-luminescent. OELD devices can easily display moving images because they have a short response time of only several microseconds. Moreover, such devices are not limited to a restricted viewing angle as other ELD devices are. OELD devices are stable at low temperatures. Furthermore, their driving circuits can be cheaply and easily fabricated because the circuits require only a low operating voltage, for example, about 5V to 15V DC (direct current). In addition, the process used to manufacture OELD devices is relatively simple.
In general, an OELD device emits light by injecting electrons from a cathode and holes from an anode into an emission layer, combining the electrons with the holes, generating an exciton, and transitioning the exciton from an excited state to a ground state. Since the mechanism by which an OELD device produces light is similar to a light emitting diode (LED), the organic electroluminescence display device may also be called an organic light emitting diode (OLED).
Recently, an active matrix OELD device where a plurality of pixel regions are disposed in the form of a matrix and a thin film transistor (TFT) is disposed in each pixel region is widely used in FPDs. An exemplary active matrix organic electroluminescent device is illustrated in FIG. 1.
FIG. 1 is a circuit diagram of an active matrix organic electroluminescent device according to the related art.
In FIG. 1, a pixel region “P” defined by a gate line 1 and a data line 3 is composed of a switching thin film transistor (TFT) “TS,” a driving TFT “TD,” a storage capacitor “CST,” and an organic electroluminescent (EL) diode “DEL.” A power line 5 is parallel to and spaced apart from the data line 3. The switching TFT “TS” includes a switching gate electrode connected to the gate line 1, and switching source and switching drain electrodes, and the driving TFT “TD” includes a driving gate electrode and driving source and driving drain electrodes. The switching source and switching drain electrodes are connected to the data line 3 and the driving gate electrode, respectively. The driving source and driving drain electrodes are connected to the power line 5 and the organic EL diode “DEL,” respectively. The organic EL diode “DEL” includes a cathode, an anode, and an organic electroluminescent layer interposed therebetween. The cathode of the organic EL diode “DEL” is grounded and the anode of the organic EL diode “DEL” is connected to the driving drain electrode. The storage capacitor “CST” is connected to the driving gate and driving source electrodes of the driving TFT “TD.”
When a gate signal is applied to the switching gate electrode through the gate line 1, the switching TFT 4 is turned ON and a data signal of the data line 3 is stored in the storage capacitor “CST” through the switching TFT “TS.” The data signal is also applied to the driving gate electrode, thereby turning the driving TFT “TD” ON. Thus, a current by a power voltage “VDD” of the power line 5 flows through a channel of the driving TFT “TD” and is transmitted to the organic EL diode “DEL.” As a result, the organic EL diode “DEL” emits light in proportion to the current density. The organic EL diode “DEL” is a current driving type that the power voltage “VDD” has a fixed value and the brightness of light is controlled by the current. Since the driving TFT “TD” may be driven by charges stored in the storage capacitor “CST” even when the switching TFT “TS” is turned OFF, the current through the organic EL diode “DEL” is persistent until a next data signal is applied. As a result, light is emitted from the organic EL diode “DEL” until a data signal of the next frame is applied.
In the organic electroluminescent device according to the related art, the power line 5 has a fixed voltage from a single power supply and is parallel to the data line 3. Since there is no means to control a power voltage “VDD” of each power line 5, each organic EL diode “DEL” emits light for an excessive time and can be overheated. As a result, lifetime is shortened and light efficiency is reduced. Moreover, since resistance value of the organic EL diode “DEL” can be changed and a response speed can be reduced, motion blurring phenomenon in which the previous image affects the next image and can cause non-uniformity of display quality can occur.